Oligomeric thiocarbonates

ABSTRACT

Oligomeric thiocarbonates such as hexathiodicarbonate salts and thioesters are used as pesticides and preservatives. Certain oligomeric thiocarbonate salts, notably quaternary ammonium hexathiodicarbonates, are prepared. The compounds are stabilized with added base and/or sulfide.

This application is a division of application Ser. No. 07/458,283, filedDec. 28, 1989, now U.S. Pat. No. 5,288,753.

TECHNICAL FIELD

This invention relates to the field of thiocarbonates and, inparticular, to oligomeric thiocarbonates, methods of making them, andtheir use as pesticides and preservatives.

INTRODUCTION

Compounds containing the structural units ##STR1## have been founduseful for a number of purposes. The chemistry of tri- andtetrathiocarbonates has been studied in some detail. See, for example,O'Donoghue and Kahan, Journal of the Chemical Society, Vol 89(II), pages1812-1818 (1906); Yeoman, Journal of the Chemical Society, Vol 119,pages 38-54 (1921); Mills and Robinson, Journal of the Chemical SocietyVol. 128(II), pages 2236-2332 (1928) and by Stone et al in U.S. Pat. No.2,893,835.

When two mols of a hydroxide base MOH (wherein the cation M is ammonium,alkali metal, or alkaline earth metal ion), and one mol each of hydrogensulfide, sulfur, and carbon disulfide are combined in a suitablesolvent, the product is the tetrathiocarbonate salt M₂ CS₄ as shown ingeneral equation (1)

    2MOH+H.sub.2 S+S+CS.sub.2 →M.sub.2 CS.sub.4 +2H2O   (1)

The tetrathiocarbonate salt comprises one mol of bound carbon disulfide.Such tetrathiocarbonate salts and methods of making and using them havebeen described in in our U.S. Pat. Nos. 4,476,113, 4,551,167, and4,726,144, which are incorporated herein by reference in their entirety.

Physical and chemical properties of thiocarbonates and a number ofmethods for making them are summarized in "Carbon Sulfides and theirInorganic and Complex Chemistry" by G. Gattow and W. Behrendt, Volume 2of "Topics in Sulfur Chemistry" A. Senning, Editor, George ThiemePublishers, Stuttgart, 1977, starting at page 154. A method of makingdialkyl hexathiodicarbonate is described. At page 176,bis(tetramethylammonium) hexathiodicarbonate solvated with carbondisulfide is described. The compound, [(H₃ C)₄ N]₂ C₂ S₆.1/2CS₂, is saidto crystallize from a solution of tetramethylammonium trithiocarbonatein methanol and carbon disulfide.

SUMMARY OF THE INVENTION

This invention comprises methods of using oligomeric thiocarbonates,processes for making certain oligomeric thiocarbonates, and certainnovel oligomeric thiocarbonates.

Oligomeric thiocarbonates release carbon disulfide, an effectivefumigant, upon decomposition. They can be used as pesticides for thetreatment of enclosed spaces, agricultural soils, trees, and crops, andas preservatives for stored cellulosic materials such as wood chips andagricultural products. They are especially useful as soil fumigants forthe control of soil-borne pests such as nematodes and fungi.

As used herein, the term "oligomeric thiocarbonate" means any chemicalcompound containing the structural unit ##STR2## wherein n is a positiveinteger, preferably from 1 to about 5, and most preferably 1. Thisstructural unit shall be referred to herein as an "oligomericthiocarbonate unit." When n is 1, the structural unit is ahexathiodicarbonate unit, having the structural formula ##STR3## Theoligomeric thiocarbonate structural unit can be present in chemicalcompounds made or used in accordance with this invention as, forexample, an anion of a salt; an acidic part of a thioester, thioamide,xanthate, or sulfenyl compound; or a ligand of a coordination compound,chelate, or other complex.

This invention provides novel processes for making certain oligomericthiocarbonates by reacting hydrogen sulfide and a strong hydroxide baseZOH (or the corresponding sulfide Z₂ S) with sulfur and carbondisulfide. In the novel processes, Z is a bulky cation, preferably atleast as bulky as a tetramethylammonium ion, sufficient to result in theformation of a reaction product comprising an oligomeric thiocarbonatestructural unit. Certain novel oligomeric thiocarbonates can be made bythe novel processes disclosed herein. Preferred among these arequaternary ammonium hexathiodicarbonates wherein at least one quaternaryammonium ion comprises at least 5 carbon atoms.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides methods of using compositions of mattercomprising oligomeric thiocarbonates. Any oligomeric thiocarbonatecompound, i.e., any compound comprising the oligomeric thiocarbonatestructural unit, can be used in accordance with this invention. Suchcompounds can be represented by the structural formula ##STR4## whereinn is an integer of at least 1, preferably from 1 to about 5, mostpreferably 1, and X and Y independently are any organic or inorganicgroups. As shown by the formula, an oligomeric thiocarbonate comprisestwo or more repeating --CS₃ -- units joined by sulfur-sulfur bonds; adimer thiocarbonate, for example, contains two --CS₃ -- units (n is 1),a trimer thiocarbonate contains three --CS₃ -- units (n is 2), and soon. The term "group" as used throughout the specification and the claimsis intended to mean any single atom as well as any assemblage of atoms,including organic and inorganic cations as well as neutral or covalentlybonded radicals. For example, a metal ion, a halogen atom, hydrogen, anethyl radical, a benzyl radical, and a tetraethylammonium ion are all"groups" as the term is used herein. X and Y can be the same ordifferent, and can be any groups capable of bonding in any way with theoligomeric thiocarbonate structural unit, whether ionically, covalently,or associatively. Both X and Y together can represent a singlepolyvalent species, such as a metal ion in a complex or an organic groupto which the oligomeric thiocarbonate unit is attached as part of a ringstructure. Preferably, X and Y are separate groups.

X and Y can be organic radicals. As used herein throughout thespecification and the claims, the term "organic radical" means anyradical which contains at least one carbon atom. An organic radical canbe derived from an aliphatic, allcyclic, or aromatic compound, and caninclude straight chain, branched chain, and cyclic structures. Anorganic radical can be, for example, a substituted or unsubstitutedalkyl, alkenyl, alkynyl, aryl, arylalkyl, or alkylaryl group, and caninclude heteroatoms such as oxygen, sulfur, nitrogen, and phosphorus. Anorganic radical can be joined to the oligomeric thiocarbonate structuralunit at a carbon atom, e.g., a carbon atom of an alkyl group, or at aheteroatom contained in the organic radical, e.g., an oxygen atom of analkoxy group, a nitrogen atom of an amino group, or a sulfur atom of amercapto group; and typically is joined with a covalent bond. Typically,the organic radical comprises from 1 to about 100 carbon atoms, e.g.,from 1 to about 50 carbon atoms, preferably from 1 to about 20 carbonatoms. More preferably, the organic radical is a hydrocarbyl grouphaving from 1 to about 8 carbon atoms, such as methyl, ethyl, n-propyl,n-butyl, sec-butyl, t-butyl, hexyl, octyl, phenyl, and benzyl, and evenmore preferably an alkyl group having from 1 to about 4 carbon atoms.

Organic cations, i.e., organosubstituted heteroatomic cations, arepreferred groups for X and Y. These can be represented by the generalformula

    R.sub.1 R.sub.2 R.sub.3 R.sub.4 Q.sup.+

wherein each of the four R groups independently is hydrogen or anorganic radical, preferably a hydrocarbyl organic radical, with at leastone R being an organic radical; and Q is a nonmetal, semi-metal ormetalloid. Each R group typically has from 1 to about 50 carbon atoms,preferably from 1 to about 24 carbon atoms, with the total number ofcarbon atoms in all four R groups preferably being a maximum of about60. Q is preferably nitrogen, phosphorus, arsenic, or antimony. Thesecations include, for example, primary, secondary, tertiary, andquaternary ammonium, phosphonium, arsonium, and stibonium cations; thequaternary cations are preferred; and quaternary ammonium cations aremost preferred.

X and Y can also be inorganic groups. The term "inorganic group" as usedherein throughout the specification and the claims means any group whichcontains no carbon atoms, and is intended to include metals,semi-metals, and metalloids as well as nonmetals such as the halogens,hydrogen, sulfur, nitrogen, and oxygen. Suitable inorganic groupsinclude metals, typically in the form of cations, including alkalimetals such as sodium and potassium, alkaline earth metals such ascalcium, barium, and strontium, and transition metals such as iron,copper, nickel, zinc, lead, and cadmium; and ammonium ion.

Each --CS₃ -- subunit of the oligomeric structural unit corresponds toand comprises one mol of bound carbon disulfide. An oligomericthiocarbonate compound thus comprises at least two mols of bound carbondisulfide, and may comprise a higher number, e.g., three, four, five, orsix mols of bound carbon disulfide. The term "bound carbon disulfide" asused in the specification and the claims means carbon disulfide that ispart of a --CS₃ -- subunit, as distinguished from unreacted carbondisulfide that may also be associated with an oligomeric thiocarbonatecompound by solvation, for example. Each mol of bound carbon disulfidein the oligomeric thiocarbonate can be released as free carbon disulfideby decomposition of the oligomeric thiocarbonate. The bound carbondisulfide can be released all at once, for example by treating theoligomeric compound with a strong acid such as hydrochloric acid. Undersome conditions, the bound carbon disulfide can be releasedincrementally. For example, certain water-soluble thiocarbonate dimers,e.g., tetramethylammonium and tetraethylammonium hexathiodicarbonates,can partially decompose in aqueous solution to release one mol of CS₂,leaving tri- or tetrathiocarbonates in solution, which can furtherdecompose at a later time to release a second mol of CS₂. Theincremental release of carbon disulfide can be advantageous forachieving prolonged pesticidal activity with the use of oligomericthiocarbonates.

The present invention also provides processes of making certainoligomeric thiocarbonates. In one embodiment, an oligomericthiocarbonate is made by reaction of sulfur, carbon disulfide, hydrogensulfide, and a strong hydroxide base having a bulky cation.Alternatively, these oligomeric thiocarbonates can be made by reactionof sulfur, carbon disulfide, and a sulfide that can be regarded as thereaction product of hydrogen sulfide and such a base.

Proper selection of the base (or the corresponding sulfide) is criticalto the formation of an oligomeric thiocarbonate in accordance with thenovel processes of this invention. The base must be a strong hydroxidebase ZOH (or corresponding sulfide Z₂ S) having a bulky cation Z,preferably a cation at least as bulky as a tetramethylammonium ion.Bases with smaller cations, such as sodium hydroxide and potassiumhydroxide, react to produce only monomeric thiocarbonates, e.g.,tetrathiocarbonates, rather than the oligomeric thiocarbonates, e.g.,hexathiodicarbonates (dimer thiocarbonates) made in accordance with theprocesses of this invention. Quaternary ammonium hydroxides arepreferred bases. One class of quaternary ammonium hydroxides can berepresented by the general formula

    R.sub.5 R.sub.6 R.sub.7 R.sub.8 NOH

wherein each of the four R groups independently is an organic radical,preferably a hydrocarbyl organic radical, more preferably an alkyl,aralkyl, or aryl radical, typically having from 1 to about 50 carbonatoms, and preferably from 1 to about 24 carbon atoms. Preferably, thefour R groups together have a total of at least 5 carbon atoms,typically from 5 to about 60 carbon atoms, more preferably from 5 toabout 20 carbon atoms. Preferred R groups include methyl, ethyl,n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, t-butyl, benzyl, andphenyl groups.

Oligomeric thiocarbonates function as fumigants principally bydecomposing to release carbon disulfide, a volatile, active toxicant. Inaddition, quaternary ammonium ions have independent biocidaleffectiveness, particularly against microorganisms such as bacteria,algae, and fungi. The presence in a quaternary ammonium ion of at leastone higher aliphatic, preferably alkyl, group having at least about 12carbon atoms enhances the biocidal activity of the ion. Thus, oligomericthiocarbonate compositions comprising higher alkyl substitutedquaternary ammonium ions are preferred when such microorganisms are tobe controlled. In this embodiment of the invention, at least one,preferably the largest, of the four R groups is an alkyl group having atleast about 12 carbon atoms, and more preferably, when all of the Rgroups are aliphatic, between about 16 and about 18 carbon atoms. Thepresence of an aralkyl group such as a benzyl group in combination witha higher alkyl group even further enhances the biocidal activity of aquaternary ammonium ion. When at least one of the R groups is an aralkylgroup such as benzyl, the largest R group is preferably an alkyl grouphaving between about 12 and about 16 carbon atoms.

Many suitable quaternary ammonium hydroxides and salts, from whichhydroxides can be prepared, for example by ion exchange, arecommercially available. Specific examples of useful quaternary ammoniumhydroxides include tetramethyl-, ethyltrimethyl-,methylethylphenylbenzyl-, methylethylpropylbutyl-, trimethyloctadecyl-,dimethyldioctadecyl-, trimethyltallow-, trimethylsoya-, trimethylcoco-,dimethyldicoco-, dimethyldi(hydrogenated tallow)-, trimethyldodecyl-,trimethylhexadecyl-, trimethylbenzyl-, dimethyldodecylbenzy-,dimethyltetradecylbenzyl-, dimethylhexadecylbenzyl-,dimethyloctadecylbenzyl-, methylbis(2-hydroxyethyl)coco-,methylpolyoxyethylenecoco-, methylbis(2-hydroxyethyl)oleyl-,methylpolyoxyethyleneoleyl-, methylbis(2-hydroxyethyl)octadecyl-,methylpolyoxyethyleneoctadecyl-, n-dodecyltetradecyldimethylbenzyl-,n-tetradecylhexadecyldimethylbenzyl-,n-dodecyltetradecyldimethylbenzyl-,n-dodecyltetradecyldimethyldichlorobenzyl-, n-octadecyldimethylbenzyl-,and n-dodecyltetradecylhexadecyldimethylethylbenzylammonium hydroxides.

Another class of quaternary ammonium hydroxides useful in the practiceof this invention consists of pentavalent nitrogen ring compounds inwhich the ring nitrogen also bears an organic radical, e.g., laurylpyridinium hydroxide and hexadecyl pyridinium hydroxide.

Similarly, strong hydroxide bases comprising other bulky cations, suchas quaternary phosphonium, arsonium, and stibonium ions, can be used. Inaddition, other hydroxides having large cations can be used, such ascesium hydroxide.

In one embodiment of the invention, a quaternary ammoniumhexathiodicarbonate, for example, tetraethylammoniumhexathiodicarbonate, is made by reacting the corresponding quaternaryammonium hydroxide, hydrogen sulfide, carbon disulfide, and elementalsulfur in a molar ratio of about 2 mols of the quaternary ammoniumhydroxide, about 1 mol of hydrogen sulfide, about 2 mols of carbondisulfide, and about 1 gram-atom of sulfur. The hydroxide is usuallyused as an aqueous solution, preferably at least about 40 weight percentin concentration. The proportion of water in the reaction medium ispreferably kept to a minimum because the product may be slightly solublein water. The reaction proceeds quickly at room temperature withevolution of heat, and is desirably conducted in a closed vessel underinert atmosphere at atmospheric pressure with cooling to keep thetemperature below the boiling point of the reaction mixture, i.e., belowthe boiling point of carbon disulfide, about 46° C. Lower and highertemperatures and pressures can be used if desired, provided the reactionmixture is kept between its freezing and boiling points. A yellowprecipitate of quaternary ammonium hexathiodicarbonate formsimmediately. Initially, the carbon disulfide and the aqueous hydroxidesolution usually form separate liquid phases. When the carbon disulfidephase disappears, the reaction is substantially complete. The reactionmixture is allowed to cool and the precipitate is recovered in acentrifuge, washed with a higher aliphatic alcohol (that is, higher thanethanol, for example, isopropanol) and then ether or a volatilehydrocarbon, centrifuged after washing and dried by flash evaporation.Quaternary ammonium hexathiodicarbonates can be made in this way inyields of at least about 25 percent of theory, usually at least about 50percent, often at least about 75 percent, and even more than about 85percent of theory.

In another embodiment, the reactants are added simultaneously andcontinuously to a well stirred, cooled reactor. A product slurry iscontinuously withdrawn and passed through a series of continuouscentrifuges, where it is sequentially centrifuged, washed with analiphatic alcohol, centrifuged, washed with ether or volatilehydrocarbon, centrifuged, and finally recovered. The product is dried byflash evaporation. The solvents are recovered by standard techniques forpurification and are reused.

In another embodiment, a quaternary ammonium trithiocarbonate is firstproduced. For example, it can be made by reaction of a quaternaryammonium hydroxide, hydrogen sulfide, and carbon disulfide in a molarratio of about 2 to about 1 to about 1, following batch or continuousprocedures such as those described above. The trithiocarbonate,preferably dissolved in water, is reacted with elemental sulfur. Ahexathiodicarbonate is produced, rather than the tetrathiocarbonate thatwould be expected based on previously published information onthiocarbonate chemistry. The equation for this reaction is

    2(R.sub.4 N).sub.2 CS.sub.3 +S→(R.sub.4 N).sub.2 (CS.sub.3).sub.2 +(R.sub.4 N).sub.2 S

wherein R₄ N represents any quaternary ammonium ion. The bis-quaternaryammonium sulfide so produced is then reacted with carbon disulfide toproduce more quaternary ammonium trithiocarbonate, which is recyledthrough the process.

The processes of the invention are preferably carried out under an inertatmosphere, i.e., in the substantial absence of oxygen, to avoidoxidative degradation of the product. The reactions are exothermic,particularly steps involving the reaction of base with hydrogen sulfide,so that sufficient cooling should be provided to prevent excessiveboiling of the reaction mixture. Any convenient temperatures andpressures can be used; at normal atmospheric pressure, temperatures inthe range of about -10° to about 50° C. are ususally satisfactory.Preferably, the temperature is maintained below about 10° C. during themost exothermic phase and is then raised somewhat, e.g., to betweenabout 20° and 30° C., to promote completion of the reaction.

The processes specifically described above involve the use of quaternaryammonium compounds. However, corresponding hydroxides andtrithiocarbonates containing phosphonium, arsonium, stibonium, and othersuitable cations can also be used to produce the correspondinghexathiodicarbonates and higher oligomeric thiocarbonates.

Oligomeric thiocarbonates prepared as described above can then beconverted to other oligomeric thiocarbonates of this invention. Usually,the conversion is carried out under non-oxidizing conditions, since itappears that the oligomeric thiocarbonate structural unit is sensitiveto oxidative degradation.

One conversion method is ion exchange. For example, a cation-exchangeresin is charged with the desired cation, such as an alkali metal,alkaline earth metal, transition metal, ammonium, organosubstitutedammonium, organosubstituted phosphonium, organosubstituted arsonium, ororganosubstituted stibonium ion, and a solution of a quaternary ammoniumoligomeric thiocarbonate in water is passed through a column of thecharged resin.

Many ion exchange materials are available commercially. These includeinorganic materials such as mineral zeolites (for example, sodalite andclinoptilolite), the greensands, and clays (for example, themontmorillonite group), and synthetic products such as the gel zeolites,the hydrous oxides of polyvalent metals (for example, hydrated zirconiumoxide), and the insoluble salts of polybasic acids with polyvalentmetals (such as zirconium phosphate). Synthetic organic cation-exchangeresins include weak-acid types based primarily on acrylic or methacrylicacid that has been cross-linked with a difunctional monomer such asdivinylbenzene, and strong-acid types based primarily on sulfonatedcopolymers of styrene and divinylbenzene. Synthetic organicanion-exchange resins of varying base strengths are based on primary,secondary, and tertiary amine functionality incorporated into a varietyof polymers, including epichlorohydrin-amine condensates, acrylicpolymers, and styrene-divinylbenzene copolymers. Usually,cation-exchange materials will be employed when it is desired to replacethe cation, e.g., the quaternary ammonium cation, used in thepreparation of an oligomeric thiocarbonate with another cation.

The stability of oligomeric thiocarbonate compositions, whether solids,solutions, or suspensions, can be enhanced by the presence of sulfideand/or polysulfide. Stabilized oligomeric thiocarbonate compositions areless prone to premature decomposition and release of free carbondisulfide. Suitable sources of sulfide can be represented by the formula

    M.sub.n S.sub.x

wherein M is selected from ammonium, alkali and alkaline earth metals,and organosubstituted heteroatomic cations; n is 2 when M is monovalentand 1 when M is divalent; and x is at least 1, usually from 1 to about5. Suitable sources include, for example, ammonium, sodium, potassium,and calcium sulfides and polysulfides; organosubstituted ammonium,phosphonium, arsonium, and stibonium sulfides, e.g., bis-quaternaryammonium sulfides, and combinations thereof. Typically, the compositioncomprises at least about 0.01 equivalent (0.005 mol) of sulfide per moleof oligomeric thiocarbonate, preferably at least about 0.02 equivalentper mol, more preferably at least about 0.04 equivalent per mol, andeven more preferably at least about 0.08 equivalent per mol. Much higherproportions of sulfide can be present without loss of stability, even asmuch as 10 equivalents or more of sulfide per mol of oligomericthiocarbonate, preferably less than about 1 equivalent per mol, morepreferably less than about 0.5 equivalent per mol. However, the higherproportions of sulfide can be useful when it is desired to introduceadditional sulfur into soil in addition to fumigating it. Sulfide can beintroduced at the time the oligomeric thiocarbonate is made, for exampleby including appropriate proportions of base and hydrogen sulfide inexcess of the stoichiometric amount required to form the oligomericthiocarbonate. Since thiocarbonates tend to decompose in the presence ofacid, preferably sufficient base is used to neutralize the hydrogensulfide. Alternatively, a source of sulfide can be admixed with theoligomeric thiocarbonate after it is made, for example when it is beingmade up into a formulation suitable for application, e.g., pellets,powder, solution, or suspension.

An excess of base is also beneficial in stabilizing the oligomericthiocarbonates. Suitable bases include ammonium and alkali metalhydroxides, and hydroxide bases comprising organosubstitutedheteroatomic cations, e.g., organosubstituted ammonium, phosphonium,arsonium, and stibonium ions. Quaternary ammonium hydroxides arepreferred. Generally, the amount of added base will correspond to about0.01, usually about 0.02, preferably at least about 0.04, and mostpreferably at least about 0.08 equivalents of base per mole ofoligomeric thiocarbonate.

Any amount of added base, sulfide, or polysulfide enhances the stabilityof the oligomeric thiocarbonates. Combinations of the described bases,sulfides, and/or polysulfides can be used to further enhance stabilityand are presently preferred. Presently, the most preferred stabilizedoligomeric thiocarbonate compositions contain added base in addition toone or more of the described sulfides or polysulfides.

Oligomeric thiocarbonates, e.g., bis(tetramethylammonium)hexathiodicarbonate, can be made in accordance with this inventionessentially free of solvation by carbon disulfide, i.e., free of carbondisulfide present by solvation.

Compositions comprising oligomeric thiocarbonates can be used asagricultural biocides in any application where tri- andtetrathiocarbonates can be used, as described, for example, in U.S. Pat.Nos. 4,476,113, 4,551,167, and 4,726,144. They can be used for thecontrol of a wide variety of plant and animal pests, including insects,rodents, fungi, nematodes, acarids, bacteria, arachnids, gastropods, andworms. They can be used in or on soil and as aerial plant pesticides fortopical treatment of trees or crops. At higher application rates, theycan also be used as herbicides for the control of undesirable plants.

Such compositions can be formulated in many ways for variousapplications in agricultural practice. They can be formulated, forexample, as powders or wettable powders, alone or admixed with carriers,extenders, coatings, and other additives; as crystalline solids; ascompressed pellets, alone or with binders; and in liquid medium, thatis, as suspensions or solutions in water or organic liquids such as oilsor solvents.

Oligomeric thiocarbonates have several advantages over the tri- andtetrathiocarbonates. They are not appreciably hygroscopic, they are lesssubject to oxidation in air, and they are more stable when in contactwith a dry substrate such as dry foliage or soil. They can be madesoluble or substantially insoluble in water, for best results indifferent applications. For example, tetraalkylammoniumhexathiodicarbonates in which the tetraalkylammonium ions have a totalof less than about 16 carbon atoms, preferably no more than about 12carbon atoms, are soluble to a useful degree in water. Those in whichthe tetraalkylammonium ions have 16 or more carbon atoms have lowsolubility and thus are more stable in the presence of moisture.

Thus, oligomeric thiocarbonates can be applied to the aerial portions ofplants, e.g., foliage, stems, fruit, or tree trunks, as a dry powder ora suspension in a non-aqueous, nonsolvent liquid carrier such as an oil,and will retain their pesticidal activity for a substantial length oftime, providing prolonged protection against fungal and bacterialinfection and acting as a contact pesticide against animal pests of allkinds.

Soil application of an oligomeric thiocarbonate composition can beaccomplished either prior to planting or after plant growth isestablished. It should be noted, however, that different plant speciesexhibit differing tolerances to chemical agents. In addition, thephytotoxicity of a chemical agent to a particular plant can be dependentupon its growth stage. Germination is not inhibited for most plant seedsafter soil treatment, and growth of established plants is usually notsignificantly altered. Some seedlings can show phytotoxicity symptoms.

The compositions can be applied in liquid medium by spraying onto thesoil surface. Injection into the soil, using a shank or knife, is also auseful method for applying the compositions. This application can eitherbe "flat," wherein the injectors are closely spaced to treat essentiallythe entire field area, or can be "localized" by spacing the injectorssuch that only the plant growing bed is treated, in bands. Similarly,solid compositions can be broadcast on the soil or, preferably,distributed in trenches or furrows and covered over with soil. The solidcompositions can also be applied in a localized manner near the rootzone of crops or trees, for example around trees under the drip line.Because the oligomers are stable in contact with dry soil, they can beapplied to the soil and allowed to remain until the soil is irrigated,either by natural rainfall or by artificial means such as sprinkler orflood irrigation. The irrigation water will move the composition intothe soil and promote the release of carbon disulfide, the actualfumigant. At the same time, the water slows the diffusion of the carbondisulfide from the soil into the air, and thus prolongs contact betweenthe carbon disulfide and soil pests.

Oligomeric thiocarbonates are effective as soil fumigants over a widerange of application rates. The particular application rate for aparticular situation depends on many factors, such as the pest or peststo be controlled, the crop to be protected and its stage of growth, soilmoisture and other conditions, and the like. Generally, the compositionsprovide beneficial effects at application rates as low as about 1 pound,preferably at least about 5 pounds, of releasable carbon disulfidecontent per acre of soil treated, and as high as about 2000 pounds,usually less than about 1000 pounds, CS₂ per acre. Typically, for theoverall fumigation of cultivated fields, the compositions are applied atrates of from about 10 to about 500 pounds, preferably less than about250 pounds, more preferably less than about 125 pounds, and mostpreferably from about 15 to about 75 pounds, CS₂ per acre. If thecompositions are applied in a localized manner, for example under thedrip lines of trees, the effective application rate can be much higherthan the average per acre, since only the soil in the root zones isactually treated.

Another example of soil application is in the planting of trees, or thereplacement of diseased or dead trees, in an orchard. When the plantinghole is dug or the old tree is removed, an oligomeric thiocarbonatecomposition can be mixed with the loose soil from the hole, which isthen placed around the roots of the new tree. This controls pests in andaround the planting hole and provides a healthier environment for thenew tree to become established.

The oligomeric thiocarbonates can be combined with other agriculturalchemicals to provide a multifunctional product. For example, they may becombined with solid or liquid fertilizers such as urea, ammonia,ammonium nitrate, calcium nitrate, etc. and other sources of plantnutrients. Since the described thiocarbonates inhibit nitrification,they reduce the rate at which ammoniacal compounds, such as fertilizers,are nitrified in the soil. Ammoniacal fertilizers are well known in theart, and as that term is used herein, it includes ammonia andammonium-containing compounds as well as ammonia and ammonium compoundformers such as urea, biuret, etc. Illustrative ammonium-containingcompounds include ammonium nitrate, ammonium sulfate, etc.

The compositions also can be used in non-soil fumigation procedures,such as in the chamber fumigation of commodities which are introducedinto commerce. In this type of procedure, acidification of a compositionor the application of heat, or both, can be used to promote a rapiddecomposition into the fumigant components. Upon termination of thefumigation procedure, vapors in the chamber can be drawn through ascrubbing system, e.g., one containing an alkaline aqueous solution, toremove the fumigant and prevent atmospheric pollution when the chamberis opened.

Oligomeric thiocarbonate compositions are also useful for thepreservation of cellulosic materials stored in bulk. Such materials, forexample, silage, wood chips, animal feed, grain, hay, straw, and thelike, are often stored for considerable periods of time in bulk underless than ideal conditions. Residual moisture in the materials, groundmoisture, and rain can promote the growth of molds, fungi, and bacteria.Heat generated by the growth of those organisms, as well as metabolicactivity of surviving cells of the stored material, further acceleratesthe deterioration of the material. Substantial loss of valuableconstituents and total biomass of the stored material often results.Treating such materials with an oligomeric thiocarbonate, such as aquaternary ammonium hexathiodicarbonate, can inhibit the metabolicactivity and growth of organisms that contribute to the loss. A fumigantcomposition can be applied simply by spraying into the product as it isbeing transported to the storage enclosure with a conveyor, auger orother device. The composition can be applied to agricultural productswhich are already in storage, by spraying onto the exposed products andsealing the storage enclosure. Preferably, when the material is storedoutdoors (e.g., wood chips), the thiocarbonate composition is applied asa suspension in a light hydrocarbon or mineral oil, which enhances thedispersion of the thiocarbonate throughout the mass, and provides somemeasure of protection against loss of thiocarbonate due to rain.Usually, the oligomeric thiocarbonate is applied at the rate of at leastabout 0.1 pound of releasable CS₂ content per dry ton of stored material(0.05 kg per metric ton), preferably between about 0.3 and about 5pounds per ton (between about 0.15 kg and about 2.5 kg per metric ton).

It is also possible to use the thiocarbonate compositions for fumigatingrooms or storage enclosures; this is accomplished by spraying the floorand walls with the composition, and sealing the space until the desiredfumigation is accomplished. As an alternative to spraying, a techniquesimilar to chamber fumigation can be used, wherein heat decomposes thecomposition in an enclosed space.

The fumigating property of compositions described herein has beenexpressed primarily in terms of the available carbon disulfide content.It should be noted, however, that other components can contribute toefficacy as a pesticide. Quaternary ammonium ion, for example, is widelyemployed for disinfecting and algicidal purposes. In addition, sulfur isvery widely used as a combination fungicide, acaricide, and insecticide,so any of the compositions of the invention which decompose to formsulfur will have similar properties in addition to the propertiesattributable to the carbon disulfide content.

The invention is further described by the following examples, which areillustrative of various aspects of the invention and are not intended aslimiting the scope of the invention as defined by the appended claims.

EXAMPLE 1

To a three-neck flask containing 150.0 g of 40 weight percent aqueoustetraethylammonium hydroxide (0.408 mol ) was added 6.85 g (0.214 mol )of sulfur powder and 6.94 g (0. 204 mol ) of hydrogen sulfide gas. Thesolution, which warmed slightly, was stirred to dissolve the sulfur.Then 32.56 g (0.428 mol) of carbon disulfide was added dropwise withmoderate stirring and cooling to maintain the temperature at or below40° C. During cooling to room temperature, an orange solid precipitated.The reaction mixture was then cooled to 0° C., and the solid wasisolated by filtration, washed twice with cold isopropyl alcohol, twicewith ethyl ether, and dried. The product was crystalline andfree-flowing. The yield was 85.9 g, about 88.5 percent based on amolecular weight of 476.

The crystals appeared to become only slightly less free-flowing onstanding overnight on a watch glass at 50 percent relative humidity.

Upon decomposition in acid, and collecting carbon disulfide in toluene,it was found that the solid contained 29.5 weight percent carbondisulfide. The nominal carbon disulfide contained in monomerictetraethylammonium tetrathiocarbonate [(C₂ H₅)₄ N]₂ CS₄, molecularweight 400, is 19 weight percent. The nominal carbon disulfide containedin the dimer, tetraethylammonium hexathiodicarbonate (C₂ H₅)₄ NCS₃ CS₃N(C₂ H₅)₄, molecular weight 476, is 31.9 percent.

EXAMPLE 2

In this example, tetramethylammonium hydroxide, hydrogen sulfide,sulfur, and carbon disulfide are reacted in a molar ratio of 2:1:4:4 toproduce an oligomeric thiocarbonate comprising 4 mols of bound carbondisulfide: ##STR5##

To a bottle containing 12.32 g (0.068 mol) of solid tetramethylammoniumhydroxide pentahydrate and 4.35 g (0.136 mol) of sulfur powder was added1.16 g (0.034 mol) of gaseous hydrogen sulfide. The solid base becameliquid upon addition of the hydrogen sulfide and almost all the sulfurdissolved. In one portion was added 10.34 g (0.136 mol) of carbondisulfide, which formed a second liquid phase. The mixture was shakenfor 30 minutes. After about 15 minutes, there was no apparent freecarbon disulfide; that is, the second liquid phase had disappeared. Themixture became almost solid with orange precipitate. The mixture waschilled, and the solid was filtered on a sintered glass crucible, duringwhich it became darker and redder. It was held under vacuum in adessicator with concentrated sulfuric acid drying agent overnight. Theproduct was a dark red viscous solid with amine odor, which showed noevident tendency to be hygroscopic. The viscous solid was madefree-flowing by the addition of 35 weight percent calcium hydroxide andtriturating with a spatula. The product was presumed to be (CH₃)₄N(CS₃)₃ (CS₄)N(CH₃)₄, molecular weight 612, theoretical carbon disulfidecontent 49.7 weight percent. The measured CS₂ content was 25-26 weightpercent.

When an alkali metal hydroxide is used as the base, the product is thealkali metal tetrathiocarbonate, and only one mol of carbon disulfideand one gram-atom of sulfur are consumed per two mols of base and onemol of hydrogen sulfide. The excess carbon disulfide and sulfur remainas a clearly visible second phase. Thus it is surprising that, when aquaternary ammonium hydroxide is used as the base, all of the excesscarbon disulfide and sulfur are consumed as shown in this example.Although the analytical result above did not confirm the formation of anoligomer containing four mols of bound carbon disulfide, it is notinconsistent with the formation of a product mixture comprising such anoligomer.

EXAMPLE 3

In several screening tests, tetraethylammonium hexathiodicarbonate hasshown a higher degree of efficacy against nematodes and fungi than asolution of sodium tetrathiocarbonate in water having the samereleasable carbon disulfide content.

Test 1

Six-inch diameter pots, each containing 1800 g of sandy soil with agrowing tomato plant, received 1500 larvae each of root-knot nematode,Meloidogyne incognita, injected uniformly around the tomato roots. Testchemical was added by trenching around the plant and covering with soil.Untreated controls received only water. All treatments were replicated 6times. The tomatoes were grown 35 days, at which time the whole plantswere harvested. The extent of root galling was indexed on a scale of 0to 5, where 5 denoted that the root system was totally galled. Theresults are summarized in Table 1.

                  TABLE 1                                                         ______________________________________                                        Treatment         Mean Galling Index                                          ______________________________________                                        Control (water only)                                                                            4.5                                                         Na.sub.2 CS.sub.4 solution,                                                   0.39 g CS.sub.2 /pot, 217 ppmw                                                                  2.0                                                         0.19 g CS.sub.2 /pot, 105 ppmw                                                                  1.7                                                         (Et.sub.4 N).sub.2 C.sub.2 S.sub.6 powder,                                    0.17 g CS.sub.2 /pot, 94 ppmw                                                                   0.8                                                         0.09 g CS.sub.2 /pot, 50 ppmw                                                                   0.9                                                         ______________________________________                                    

Test 2

Sterilized test soil was uniformly infested with pythium, a water-mold.The soil was split into 6 replicates, 500 g each, in 1-liter beakers,and incubated 24 hours. A 200 g subsample from each beaker was thenplaced in an aluminum tray in a layer about 1/2 inch deep. To each traywere added 20 ripe tomatoes, 120 in all, as bait for pythium. After 4days, each tomato was inspected for a watery, rotted appearance at thearea of contact with soil. The results are presented in Table 2.

                  TABLE 2                                                         ______________________________________                                        Treatment       % of Fruit Infected                                           ______________________________________                                        Uninfested control                                                                            0.0                                                           Infested, untreated                                                                           16.7                                                          Na.sub.2 CS.sub.4 solution,                                                   100 ppmw CS.sub.2 /soil                                                                       12.5                                                          400 ppmw CS.sub.2 /soil                                                                       20.0                                                          (Et.sub.4 N).sub.2 C.sub.2 S.sub.6 powder                                     100 ppmw CS.sub.2 /soil                                                                       6.7                                                           400 pmw CS.sub.2 /soil                                                                        7.5                                                           ______________________________________                                    

Test 3

Sterilized test soil was uniformly infested with phytophthora, awater-mold. The soil was split into 500 g (dry weight basis) portions in1-liter beakers. Treatments were added to the beakers by opening 4 holesin a uniform pattern, adding test chemical evenly among the holes, andclosing completely. Treatments were replicated 6 times. The treatedsoils were incubated 24 hours, at which time each was mixed thoroughly.Three equal subsamples from each were placed in aluminum trays in layersabout 1/2 inch deep. To each tray were added 12 green tomatoes as baitfor phytophthora. After 2 and 3 days, each tomato was inspected forbrown spots, a symptom of infection. From the number of spotted tomatoesthe disease incidence (% of fruit) was calculated. In addition, after 4days, the average percentage of fruit surface browned was estimated, andpresented as disease severity (% of surface area). The results arepresented in Table 3.

                  TABLE 3                                                         ______________________________________                                                   Disease Incidence                                                  Treatment    Day 2    Day 3    Disease Severity                               ______________________________________                                        Untreated,                                                                    uninfested    0        0        0                                             infested     100      100      79                                             Na.sub.2 CS.sub.4 solution                                                    100 ppmw CS.sub.2 /soil                                                                    100      100      72                                             400 ppmw CS.sub.2 /soil                                                                     22      100      54                                             (Et.sub.4 N).sub.2 C.sub.2 S.sub.6                                            100 ppmw CS.sub.2 /soil                                                                     56      100      37                                             400 ppmw CS.sub.2 /soil                                                                     11       77      12                                             ______________________________________                                    

The oligomeric thiocarbonate was more effective than sodiumtetrathiocarbonate, on an equivalent CS₂ basis, in reducing theincidence and/or severity of the infection.

While particular embodiments of the invention have been described, itwill be understood, of course, that the invention is not limitedthereto, since many obvious modifications can be made, and it isintended to include within this invention any such modifications as willfall within the scope of the appended claims.

Having now described the invention, we claim:
 1. A compound representedby the formula ##STR6## wherein X and Y independently are selected fromorganic cations; n is at least 2; the organic cations have the formulaR₁ R₂ R₃ R₄ Q⁺ ; R₁, R₂, R₃, and R₄ are independently selected from thegroup consisting of hydrogen and C₁ -C₅₀ organic radicals selected fromthe group consisting of alkyl, aryl, arylalkyl, and alkylaryl groups,provided that at least one of R₁, R₂, R₃, and R₄ is an organic radical;and Q⁺ is selected from the group consisting of ammonium, phosphonium,arsonium, and stibonium.
 2. A composition comprising:(a) a compoundrepresented by the formula ##STR7## wherein X and Y independently areorganic cations; n is at least 1; the organic cations have the formulaR₁ R₂ R₃ R₄ Q⁺ ; R₁, R₂, R₃, and R₄ are independently selected from thegroup consisting of hydrogen and C₁ -C₅₀ organic radicals selected fromthe group consisting of alkyl, aryl, arylalkyl, and alkylaryl groups,provided that at least one of R₁, R₂, R₃, and R₄ is an organic radical;and Q⁺ is selected from the group consisting of ammonium, phosphonium,arsonium, and stibonium; and (b) a stabilizing agent selected from thegroup consisting of sulfides having the formula M_(m) S_(y) and bases,wherein M is selected from the group consisting of ammonium, alkalimetals, alkaline earth metals, and organosubstituted heteroatomiccations; m is 2 when M is monovalent; m is 1 when M is divalent; and yis at least
 2. 3. The composition of claim 2 wherein R₁, R₂, R₃, and R₄are independently selected from the group consisting of hydrogen and C₁-C₂₄ organic radicals.
 4. The composition of claim 2 wherein R₁, R₂, R₃,and R₄ are independently selected from the group consisting of hydrogenand C₁ -C₂₀ organic radicals; and the organic radicals are selected fromthe group consisting of alkyl, aryl, arylalkyl, and alkylaryl groups. 5.The composition of claim 2 wherein R₁, R₂, R₃, and R₄ are independentlyselected from C₁ -C₈ organic radicals; and the organic radicals areselected from the group consisting of alkyl, aryl, arylalkyl, andalkylaryl groups.
 6. The composition of claim 2 wherein R₁, R₂, R₃, andR₄ are independently selected from C₁ -C₄ alkyl radicals.
 7. Thecomposition of claim 2 wherein R₁, R₂, R₃, and R₄ are independentlyselected from C₁ -C₄ alkyl radicals and Q⁺ comprises ammonium.
 8. Thecomposition of claim 2 wherein the compound comprisesbis(tetraalkylammonium) hexathiodicarbonate having less than about 16carbon atoms per tetraalkylammonium ion.
 9. The composition of claim 2wherein the compound comprises bis(tetraalkylammonium)hexathiodicarbonate having at least about 16 carbon atoms pertetraalkylammonium ion.
 10. The composition of claim 2 wherein thecompound comprises bis(tetraethylammonium) hexathiodicarbonate.
 11. Thecomposition of claim 2 wherein the stabilizing agent comprises at leastone of the sulfides.
 12. A compound represented by the formula ##STR8##wherein n is at least 1; X and Y independently are organic cationshaving the formula R₁ R₂ R₃ R₄ Q⁺ ; R₁ is selected from the groupconsisting of hydrogen, n-propyl, isopropyl, n-butyl, isobutyl,sec-butyl, t-butyl, benzyl, and phenyl; R₂ is selected from the groupconsisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl,sec-butyl, t-butyl, benzyl, and phenyl; R₃ is selected from the groupconsisting of hydrogen, methyl, ethyl, n-propyl, isopropyl, n-butyl,isobutyl, sec-butyl, t-butyl, benzyl, and phenyl; R₄ is selected fromthe group consisting of hydrogen, methyl, ethyl, n-propyl, isopropyl,n-butyl, isobutyl, sec-butyl, t-butyl, benzyl, and phenyl; and Q⁺ isselected from the group consisting of ammonium, phosphonium, arsonium,and stibonium.
 13. A compound represented by the formula ##STR9##wherein n is at least 1; X and Y independently are organic cationshaving the formula R₁ R₂ R₃ R₄ Q⁺ ; R₁, R₂, R₃, and R₄ are independentlyselected from the group consisting of hydrogen and C₁ -C₅₀ organicradicals selected from the group consisting of alkyl, aryl, arylalkyl,and alkylaryl groups, provided that at least one of R₁, R₂, R₃, and R₄is an organic radical; and Q⁺ is selected from the group consisting ofphosphonium, arsonium, and stibonium.
 14. An agricultural compositioncomprising:(a) a compound represented by the formula ##STR10## wherein nis at least 1; X and Y independently are organic cations having theformula R₁ R₂ R₃ R₄ Q⁺ ; R₁, R₂, R₃, and R₄ are independently selectedfrom the group consisting of hydrogen and C₁ -C₅₀ organic radicalsselected from the group consisting of alkyl, aryl, arylalkyl, andalkylaryl groups, provided that at least one of R₁, R₂, R₃, and R₄ is anorganic radical; and Q⁺ is selected from the group consisting ofammonium, phosphonium, arsonium, and stibonium; and (b) a fertilizer.15. The composition of claim 2 wherein n is an integer from 1 to
 5. 16.The composition of claim 2 wherein n is
 2. 17. The composition of claim2 wherein n is 1.